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Sinclair ZX81

Sinclair ZX81 vintage computer

ZX81 Gameplay Videos


CPU: Z80 @ 3.25 MHz (or NEC µPD780C-1 equivalent)
Display: 64 by 48 black and white. Some very clever machine code was able to generate 256 × 192 by overriding the interrupt service routine.
Memory: 1K RAM (expandable)
Software Media:Cassette.

Value: £40 - £50. It may be old, but isn't as rare as you'd think.

ZX81 Game Videos





Sinclair ZX81 Links:

ZX81 Kit

Planet Sinclair


System Details:

The 1981 released successor to the Sinclair ZX80, the ZX81 is basically an update of the ZX80 and designed to be as small, simple and cheap as was physically possible with the technology of the day.
It sold for £69.95 pre-assembled, or £49.95 in kit form. This was incredibly cheap, even compared to its nearest competitor, the VIC-20, which sold for $260, while other contemporaries made by the likes of Apple and IBM were selling for over £600 / $1000.

The Sinclair ZX81 has no moving parts, necessitating the use of a membrane keyboard and the exclusion of an on/off switch. It has only 4 chips... the CPU (Z80A @ 3.5MHz), 1k RAM chip (expandable to 16k with an external RAM pack), 8k ROM chip and an "Uncommitted Logic Array" ULA chip, which pretty much ties everything together (this combined the functions of 18 chips on the ZX80 into one chip). This overall chip count compares very favourably with the 44 chips of the Tandy TRS-80.
Data storage is achieved by the use of magnetic cassette tapes in a normal domestic tape recorder.

ULA functions include synchronising the screen display, generating a 6.5MHz clock (double that of the CPU clock speed), audio in and out for the tape drive, keyboard operation, and controlling system timing.
One of the quirks of the ULA is that it uses the same pin for video output as it uses for tape audio input. This has the effect of filling the screen with audio interference when loading data from tape, generating the jagged black horizontal bands that users became familiar with whenever loading a program.

In some ways, though probably not intentional, this effect became a positive feature, as the thickness of the bands can be used as a guide to volume settings of the tape recorder.
If the black lines are too thick, the volume is too high.

An improvement made over the ZX80 is the inclusion of the SLOW video mode.
The ZX80 is incapable of maintaining a video signal while doing anything else, be it running program code to receiving input from the keyboard.
This has the effect of making the display very flickery.

On the ZX81, to get around this, the SLOW mode uses the majority of its processor time in maintaining the display, basically running code or accepting input while the video beam is at the top or bottom of the screen. The trade off of this is that programs running in BASIC are extremely slow, necessitating the use of machine code for creating fast responsive games.

Brief History:

Jim Westwood, Sinclair's chief engineer began development of the ZX81 before the ZX80 had even gone to market. He worked on reducing the number of components by developing a general purpose chip (ULA) which was produced for Sinclair by Ferranti.
The ZX81 is alleged to suffer overheating issues due to Sinclair using all of the logic gates in the ULA, when they were only supposed to use 70% of them. I personally have never experienced this problem.

The ZX80s ROM was improved for the ZX81, and included new maths functions, including the ability to work with floating points, where the ZX80 could only work with whole numbers. Early models were found to have a bug in them, which gave inaccurate results when calculating square roots, forcing Sinclair to offer replacements to many customers.

Manufactured in Dundee, Scotland by Timex Corporation, the ZX81 was launched on 5th March 1981 and was accompanied by a very impressive manual which instructed the user on not just how to use the computer, but how to program its built-in BASIC programming language.
Though the system sold very well, firstly through mail order and then from high street stores such as W.H.Smith, Boots, Currys and John Menzies (selling in excess of 500,000 units), reliability was an issue, and Sinclair's after sales service developed the reputation of being entirely nonexistent. Broken machines sat in corners for months, waiting to be repaired or replaced.

It's worth mentioning that the ZX81 was entered into the BBC competition to provide a computer for use in an upcoming BBC TV program. It went up against the Grundy New Brain (which was withdrawn from the competition) and the Acorn Proton (which at that point didn't even exist in prototype form).
It's a matter of history that Acorn won the competition, producing the computer that we now love and know as the Acorn BBC Micro. Clive Sinclair was needless to say not at all happy with the result, given that not only did his system actually exist, it already had an established user base.
To be fair to Acorn though, despite being vastly more expensive, their computer did turn out to be a much more rugged, capable and generally deserving system, and was certainly more suitable for the role it soon developed in the classrooms of Britain.

Being an incredibly limited machine in its basic form, a thriving market for add-ons and peripherals soon built up, with such items as rubber keyboard attachments, printer interfaces, and the legendary 16K RAM pack all being very popular items.
The design of the expansion port on the back of the ZX81 was such (it was simply a bare edge connector) that the almost mandatory 16K RAM packs had a very hard time holding a solid connection, resulting in a new term for computing unreliability. The expression "memory wobble" came into common parlance in the early 80s, and still produces a wry smile when spoken among those of a certain age and geeky inclination.

While both authorised and unauthorised variations and clones of the ZX81 can be found around the world, the most well known versions are those made by Timex in the USA.
The Timex Sinclair 1000 is basically a straight copy of the ZX81, albeit with an extra 1k of RAM, giving it a grand total of 2k.
The Timex Sinclair 1500 on the other hand did away with the membrane keyboard, and featured rubber keys and a case very similar to that of the Sinclair Spectrum. Another similarity was the inclusion of 16k of RAM, again, the same as the earliest Spectrum. This made the TS1500 a very good half way house between the original ZX81 and the new Sinclair Spectrum.

The TS1000 initially sold well in the US, but Timex made the mistake of not making the 16k RAM packs readily available, so users soon abandoned the system as being too limited to be useful.
The TS1500 on the other hand, while being an adequately functional machine came to market too late, as by this time there were already other affordable computers available that were more capable.

Buy a or a for sale on ebay.

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